Recently, the number of people suffering from various circulatory system diseases, including heart infarction and brain infarction, has been on the rise, thus making it more and more urgent to prevent and treat these diseases.
The pathopoiesis of heart or brain infarction is closely correlated to arterial sclerosis. More specifically, if an atheroma is created on the arterial wall or if no arterial cells are produced anymore due to various factors such as elevated blood pressure, then the artery loses its elasticity to become hard and fragile. Also, if the blood vessel is clogged up where the atheroma has been created or if a vascular tissue covering the atheroma has ruptured, then the atheroma will move itself into the blood vessel to clog up the artery elsewhere or to rupture the hardened portions of the artery. As a result, these diseases are caused. That is why it is important to diagnose the arterial sclerosis as early as possible to prevent or treat these diseases.
In the prior art, the lesion of arterial sclerosis is diagnosed by directly observing the inside of the blood vessel with a vascular catheter. However, this diagnosis needs to be carried out with a vascular catheter inserted into the blood vessel of a testee, thus imposing a heavy load on him or her. For that reason, the vascular catheter observation is usually adopted to locate the lesion of arterial sclerosis in a patient who is already known to suffer from that disease but has never been used to make a medical checkup on a supposedly healthy person.
A checkup may be easily made without imposing excessively heavy load on a testee if the index of cholesterol, which is one of major causes of arterial sclerosis, or the blood pressure is measured. However, neither of these values directly indicates the degree of advancement of arterial sclerosis.
Also, if the arterial sclerosis can be diagnosed early enough to administer some medicine to its patient, then the disease can be treated effectively. However, it is said that once the arterial sclerosis has advanced to a certain degree, the farther advancement of that disease can be checked with the administration of medicine but it is difficult to repair the hardened artery completely.
That is why a method or apparatus for determining the degree of advancement of arterial sclerosis at an early stage of that disease without imposing too much load on a testee is now in high demand.
Meanwhile, an ultrasonic diagnostic apparatus or an X-ray diagnostic apparatus has been used in the prior art as a noninvasive medical apparatus that imposes only a light load on a person under test. Specifically, by irradiating the testee with an ultrasonic wave or an x-ray that has been produced externally, shape information or information about the variation in the shape of his or her internal body with time can be acquired without causing pain to him or her. When the information about the variation with time (i.e., mobility information) in the shape of an object under test in his or her body can be obtained, the attribute information of the object can be obtained. That is to say, the vascular elastic property of the organism can be known and the degree of advancement of the arterial sclerosis can be detected directly.
Among other things, the ultrasonic diagnosis is superior to the X-ray diagnosis because the ultrasonic diagnosis can be made just by putting an ultrasonic probe on a person under test. That is to say, in the ultrasonic diagnosis, there is no need to administer a contrast medium to the person under test and there is no concern about potential X-ray exposure, either.
Recently, however, some ultrasonic diagnostic apparatuses can have significantly improved measuring accuracy thanks to remarkable advancement of electronic technologies. As a result, ultrasonic diagnostic apparatuses for measuring the very small motion of a vital tissue have been developed. For example, Patent Document No. 1 discloses a technique of tracking an object of measurement highly accurately by analyzing the amplitude and phase of an ultrasonic echo signal by a restricted minimum square method. This technique is called a “phase difference tracking method”. According to this technique, vibration components of a vascular motion, having an amplitude of several micrometers and a frequency of as high as several hundreds of Hz, can be measured accurately. Thus, it was reported that the thickness variation or strain of the vascular wall could be measured highly accurately on the order of several microns.
By adopting such a high-accuracy measuring technique, the two-dimensional distribution of the elastic property of the arterial wall can be plotted in detail. For example, Non-Patent Document No. 1 shows an example of presenting the two-dimensional distribution of the elasticity of the iliac bone arterial vascular wall as an image superposed on a B-mode tomogram.
It is known that the hardness of the arterial wall is not uniform but has some distribution. That is why in diagnosing the arterial sclerosis, it is important to know the exact hardness distribution of the arterial wall. According to the method disclosed in Non-Patent Document No. 1, the elasticity, which is a characteristic quantity showing the degree of advancement of the arterial sclerosis, is presented two-dimensionally, and therefore, the hardened portion of the arterial wall can be located accurately.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 10-5226        Non-Patent Document No. 1: Hiroshi Kanai et al., “Elasticity Imaging of Atheroma with Transcutaneous Ultrasound Preliminary Study”, Circulation, Vol. 107, pp. 3018-3021, 2003        